How do you add emission to singular faces
With the object in Edit mode:




Select the faces you want to be emissive.
Create a new material slot for your object, if necessary, (+ on the right)
Either create a new emissive material (+ bottom center to copy and overwrite the material in the selected slot, or New if the slot is empty), or if the material is already created, select it from the dropdown list (below the Assign button)
Hit the Assign button.